Genetic components of heat stress in finishing pigs: Development of a heat load function

¹ Department of Animal and Dairy Science, University of Georgia, Athens, 30602-2771; Norsvin, Pb 504, 2304 Hamar, Norway
² Department of Animal and Dairy Science, University of Georgia, Athens, 30602-2771
³ Department of Animal and Dairy Science, University of Georgia, Athens, 30602-2771; Departamento de Producción Animal, Universidad de León, León, 24071, Spain
⁴ Smithfield Premium Genetics Group, P.O. Box 668. Rose Hill, NC 28458

^* To whom correspondence should be addressed. E-mail: Birgit.zumbach{at}norsvin.no.

	Abstract

The objective of this study was to quantify the effect of heat stress during the life of a pig on its final weight, as a first step towards a genetic evaluation for heat tolerance. Data included carcass weights of 23,556 crossbred pigs (Duroc x [Landrace x Large White]) raised on 2 farms in North Carolina, and harvested from May 2005 through December 2006. Weather data were available from a nearby weather station. Lifetime of a pig was assumed to be partitioned into 2 periods. During an initial period, the effect of heat stress was assumed to be negligible or compensated for later. During the second period ending in harvesting, the average daily gain was assumed to be affected linearly by heat load. Weekly heat load was calculated as degrees of average temperature-humidity index (THI) in excess of a threshold (18°C). The total heat load (H) was the sum of weekly heat loads during the second period. During the months of January to May H was 0; H reached a peak in September. The final weight during the peak of heat stress decreased about 6 kg compared to weight during months of non heat stress. Weekly and monthly averages of carcass weight generally moved similarly to H. However, there were large fluctuations unrelated to H; the fluctuations were different on the 2 farms. The model included the effects of farm-year of harvest, gender, age at harvest, and H, where age at harvest and H were linear regressions. In analyses, the threshold was varied from 16 to 20°C, and the second period was varied from 8 to 16 wk. The highest R² (10.4%) was at the threshold of THI=18°C for a period of 10 wk. Varying the threshold and the length of time reduced R² less than 1%. LS means of year-month and year-week of carcass weight were calculated using a model with the fixed effects farm-year-month or farm-year-week of harvest, gender, and age at harvest (linear covariate), and the random effect of birth litter. Changes in weight of finisher pigs due to heat stress can be quantified by H during the last 10 wk of the life of the pig.

Key Words: carcass weight, heat stress, pig